The present invention relates to a refrigeration cycle used in an air conditioning apparatus for vehicles and, in particular, to a thermostatic expansion valve included in the refrigeration cycle.
Such a thermostatic expansion valve in an earlier technology is shown in FIG. 4. The thermostatic expansion valve includes an expansion valve unit 2 and a closing member 3 which are contained in a valve casing 1. More specifically, in a casing 1 there are provided a high-pressure chamber 10 and a low-pressure chamber 11 which serve as a refrigerant passage directing to a evaporator 4 for a high pressure refrigerant which is discharged from a compressor discharging chamber, low pressure passages 12 which serve as a passage directing to a compressor suction chamber for a low pressure refrigerant which is discharged from the evaporator 4, and a valve unit insertion portion 13 which is disposed between the low pressure passages 12. The closing member 3 is located at an upper portion of the valve unit insertion portion 13 such that an end of the expansion valve 2 is adaptable by the use of engagement member.
The expansion valve unit 2 has a valve seat 200a which is located to form a port 200b in the high-pressure chamber 10 of the casing 1, a valve casing 200 disposed at a center of the casing 1 to close a passage between the low-pressure chamber 11 and the valve unit insertion portion 13, a valve body 201 which is contacted with and spaced from the valve seat 200a to open/close a passage directing to the evaporator 4 through the valve seat 200a, the port 200b and the low-pressure chamber 11, a spring 203 for biasing the valve body 201 toward a valve-closing direction (an upward direction in the illustration of FIG. 4) through a guide member 202, and an adjustment screw 204 for adjusting a pressing force of the spring 203. Further, there is disposed a temperature sensing portion 205 which is disposed in the valve unit insertion portion 13 of the casing 1 such that an end portion of the temperature sensing portion 205 is mounted to the closing member 3 and which is disposed in the midst of the low pressure passage 12 directing from the outlet portion of the evaporator 4 to the suction chamber of the compressor and, in addition, a diaphragm 206 which is displaced in accordance with pressure difference between the inner pressure of the temperature sensing portion 205 and the pressure of the outlet of the evaporator 4, a transmission rod 207 which is displaceably supported to the valve casing 200 such that one end thereof is contacted with the diaphragm 206 and the other end is provided with the valve body 201 so that the valve body 201 is opened/closed in accordance with the displacement of the diaphragm 206, and a spring 208 for urging the transmission rod 207 toward the diaphragm 206.
The expansion valve unit 2 has a passage 200c at the valve casing 200 so that the diaphragm 206 receives, or effected by, the pressure from the evaporator 4 by the passage 200c.
Within the temperature sensing portion 205 which is exposed to the refrigerant from the outlet of the evaporator 4, a refrigerant (R134a) and an adsorbent (oil) is sealed therein, and the pressure in the temperature sensing portion 205 is set to be varied in accordance with the temperature of the refrigerant from the outlet of the evaporator 4.
By the structure described above, there is relationship as indicated below: EQU Fd=(Pd-Pe).multidot.Sd-(Pout-Pe).multidot.Sr-f1 ,
and EQU Fb=f2+(Pin-Pout).multidot.Sb
wherein:
Fd is a pressing force for urging the diaphragm 206 toward the valve body 201; PA1 Fb is a force effected in the valve-closing direction of the valve body 201; PA1 Pd is a pressure in the temperature-sensing portion 205; PA1 Pe is a pressure at the outlet of the evaporator 4; PA1 Pin is a pressure at the inlet of the expansion valve; PA1 f1 is a force of the spring 208; PA1 f2 is a force of the spring 203; PA1 Sd is an effective area of the diaphragm 206; PA1 Sb is a sealing area of the valve body 201; PA1 Sr is a sectional area of the transmission rod 207.
Pout is a pressure at the outlet of the expansion valve;
As a consequence, the valve body is set to be opened in case that the condition Fd&gt;Fb is satisfied.
FIG. 5 is a graph which shows the "temperature (.degree. C.)--pressure (kg/cm.sup.2 G)" characteristics under the inlet pressure conditions of the thermostatic expansion valve.
In FIG. 5, the characteristic C1 with respect to the expansion valve represents a linear line which shows that a pressure proportionally increases as the elevation of the temperature, whereas the characteristic C2 with respect to the refrigerant (R134a) represents a curve which shows that a pressure gradually varies and increases as the elevation of the temperature. As seen from FIG. 5, it is prescribed that the characteristic Cl extends across the characteristic C2.
Namely, in comparison between characteristic C1 and characteristic C2, if temperatures are compared with reference to pressure elevation up to 2.0 kg/cm.sup.2 G, the temperature of characteristic C1 represents 0.degree. C. whereas the temperature of characteristic C2 represents a temperature value slightly higher than 0.degree. C. However, if temperatures are then compared with reference to pressure elevation up to 2.7 kg/cm.sup.2 G, the temperature of characteristic C1 represents 10.degree. C. whereas the temperature of characteristic C2 represents a temperature value lower than 10.degree. C. by .DELTA.T. Thus, a relationship of the temperatures relative to the pressure is reversed at a temperature above 0.degree. C. and around 1.2.degree. C. to form a break-even or cross-over point. This is aimed to obtain restriction of hunting of an expansion valve especially at a low and middle temperature range and returning of the refrigerant (including an oil) to the compressor, because the compressor is in a continuous operation to a low outdoor temperature range and a circulation amount of the refrigerant is extremely reduced in this region.
FIG. 6 shows the "pressure of the expansion valve inlet (kg/cm.sup.2 G)--static heating degree (K)" characteristics under the condition that temperature of the temperature sensing portion 205 of the thermostatic expansive valve is made constant.
In FIG. 6, the static heating degree increases as elevation of the pressure of the expansion valve inlet. This will further show that an expansion valve inlet pressure is effected in the valve closing direction of the valve body 201, and as elevation of the expansion valve inlet pressure, a force Fb effecting towards the valve body 201 is increased and, therefore, a force Fd which effects the diaphragm 206 (that is, a pressure Pb in the temperature sensing portion 205) is required to be increased for the increase of force Fd, and that the valve body 201 can be opened by satisfying these conditions described above.
In the thermostatic expansion valve described above, the valve body has operation strongly received with influence of pressure in the refrigerant passage. It is assumed as a particular case that the valve body is not opened unless the pressure in the temperature sensing portion is increased. In the particular case, there is a problem that an appropriate operational condition is not maintained.